Glow plug provided with a pressure sensor

ABSTRACT

A glow plug including a tubular body with elements for fixing it into a bore, an intermediate tubular part arranged inside the tubular body, a finger disposed inside the intermediate tubular part with a heating electrode, and a pressure sensor connected to the intermediate tubular part, wherein a face of the sensor facing the finger is pressed thereto.

The present invention concerns a glow plug comprising a pressure sensor

BACKGROUND OF THE INVENTION

In an internal combustion engine, in particular a Diesel engine, it hasbeen noted that knowledge of the value of the pressure within eachcylinder makes it possible to better control the progress of thecombustion in the engine. This information is then used to regulate theinjection of fuel in each of the cylinders. The polluting emissions ofthe engine may thus be reduced and the consumption optimized.

In an engine of Diesel type, each cylinder comprises a glow plug whichmakes it possible to heat the inside of the corresponding combustionchamber, in particular on starting the engine. This glow plug isdisposed in a threaded bore which passes through the cylinder head ofthe engine. IT is already known to the person skilled in the art tointegrate a pressure sensor into a glow plug. In this way, the boreformed in the cylinder head to receive the glow plug also serves toaccommodate a device for measuring the pressure within the combustionchamber. Forming an additional bore in the cylinder head is thusavoided. Forming such a bore would create numerous problems due inparticular to the lack of space as well as problems of cost,fluid-tightness, accessibility, etc.

A glow plug generally comprises a tubular body having on its outersurface a threaded portion enabling it to be fixed into a correspondingbore formed in the cylinder head. One portion of that body is thenlocated within the combustion chamber and the other portion outside it.The portion within the combustion chamber bears a finger within which islocated a heater electrode. This electrode is supplied by a core whichpasses through the tubular body of the glow plug. The portion outsidethe combustion chamber is also termed glow plug head. At that head, thecore is connected to a source of electrical energy. It is also known tosituate a pressure sensor at that head. This sensor then measures thepressure exerted on the finger of the glow plug. The principle is tomeasure the force exerted on the finger by the ambient pressure in thecombustion chamber by a measurement of the stresses between the body ofthe glow plug and the finger. FIG. 1 appended hereto represents anembodiment of a glow plug of the prior art provided with a pressuresensor.

In this embodiment of the prior art, described in more detail below, thepressure sensor is disposed between the body of the glow plug on whichit bears and a nut fastened to the upper end of the core supplying thepre-heating electrode with electrical energy. In this embodiment, thecore passes through the sensor.

Such a glow plug of the prior art makes it possible to measure thepressure exerted within the combustion chamber on the glow plug finger.However, other signals become superposed on the signal corresponding tothe pressure measured. To make a correct measurement of the ambientpressure within the combustion chamber of the cylinder considered, thebackground noise perturbing the pressure measurement should be excluded.This noise arises for example from the deformations of the cylinder headin which the glow plug is mounted.

SUMMARY OF THE INVENTION

The present invention is thus directed to providing a glow plug having apressure sensor for which the signal corresponding to the pressuremeasured is not perturbed by parasite signals.

To that end, it provides a glow plug comprising:

-   -   a tubular body having means enabling it to be fixed into a bore,    -   a finger arranged inside the body of the glow plug and receiving        a heater electrode, and    -   a pressure sensor.

According to the invention this glow plug further comprises anintermediate tubular part; the finger is disposed within theintermediate tubular part; the intermediate tubular part is mounted inthe tubular body; the sensor is connected by its face that faces awayfrom the finger to the intermediate tubular part, and the face of thesensor facing the finger bears on the latter.

The fact of providing the glow plug with an intermediate tubular partmounted as stated above makes it possible to decouple the finger and thebody of the glow plug. The pressure sensor may then measure the stressesarising from the thrust exerted by the ambient pressure in thecorresponding combustion chamber without being hindered by the stressesexisting at the bore in which the glow plug is fixed and which arepartly transferred to the body of that glow plug.

The invention also provides for the intermediate tubular part to befastened to the tubular body for example in the vicinity of the end ofthe tubular body adapted to be located in the combustion chamber of thecorresponding engine. This zone of the tubular body is not in generalsubject to external stresses and the intermediate tubular part thus doesnot undergone such stresses.

In a preferred embodiment, the intermediate part is fixed to the tubularbody by an interference fit. In this way, the fixing to the tubular bodyis excellent. Fixing by welding may also be envisaged here. Similarly,the finger may be fixed to the intermediate part by an interference fitor by welding.

For better sensitivity for the measurement of pressure at the pressuresensor, the intermediate tubular part has a deformation zone between afirst fixing zone in which the finger is fastened to the intermediatetubular part and a second fixing zone in which the intermediate tubularpart is fastened to the tubular body. This deformation zone gives thefreedom of movement to the finger and avoids creating tension betweenthe intermediate tubular part and the finger which would be detrimentalto the measurement of pressure between those two parts.

To transmit the forces exerted by the pressure of the combustion chamberon the finger of the glow plug, one embodiment provides that a spacer bedisposed between the finger and the corresponding face of the sensorwithin the intermediate tubular part. This spacer must be rigid in orderto be able to pass on high stresses. Furthermore, it must not perturbthe measurement of the pressure by its vibrations. It thus has its ownvibration frequency which is very different from the bandwidth of thepressure sensor. To satisfy these demands, the spacer is for exampleformed from a ceramic material.

The spacer referred to above may take the form of an elongate tubularbush of which the outer diameter is less than the inner diameter of theintermediate tubular part and terminates at the end adapted to come intocontact with the pressure sensor with a frusto-conical portion so as topresent a bearing surface adjacent to the sensor substantiallycorresponding to the surface of the sensor. In this embodiment, a singlepart is disposed between the finger and the sensor. This is of coursefavorable to good transmission of the forces but also enables easierassembly of the glow plug.

In the glow plug according to the invention, the sensor is preferablyplaced in a housing provided for that purpose in the intermediatetubular part, adjacent to the opposite end of that part from the endadapted to receive the finger and the sensor is for example connected tothe intermediate tubular part by a nut having a screw thread on itsouter periphery engaged in a tapping made in the inner end of theintermediate tubular part. In the case in which a spacer is providedbetween the finger and the sensor, the nut may clamp the sensor againstthat spacer.

The present invention also concerns an internal combustion engine, inparticular an engine of Diesel type, characterized in that it comprisesa glow plug as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Details and advantages of the present invention will appear more clearlyfrom the following description, made with respect to the accompanyingdrawings in which:

FIG. 1 represents a glow plug of the prior art,

FIG. 2 represents a side view of a glow plug according to the invention,

FIG. 3 represents the glow plug of FIG. 2 in longitudinal cross-section,and

FIG. 4 is an exploded perspective view of the glow plug represented inFIGS. 2 and 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents a glow plug of the prior art in longitudinalcross-section comprising a pressure sensor so as to be able to measurethe pressure in an engine cylinder. This glow plug 2 is mounted in aconventional manner in an engine 4, of Diesel type, and moreparticularly in a cylinder head of that engine. It comprises a body 8, afinger 10, a core 12 and a pressure sensor 14.

The body 8 is adapted to be fixed to the engine by screwing To that end,the cylinder head 6 comprises a threaded bore passing through it andopening into a combustion chamber of said engine 4. Concerning the body8, this has a screw thread 16 on its other surface corresponding to thebore formed in the cylinder head 6. When the screw thread 16 cooperateswith the threaded bore of the cylinder head 6, the glow plug 2 being inits mounted position in the engine 4, a portion of the body 8 extendsinwardly of the engine 4, that is to say towards the combustion chamber,whereas another portion extends outwardly the engine.

The body 8 is a tubular body of steel within which are housed inparticular the finger 10 and the core 12. The finger 10 projects fromthe tubular body 8 inside the combustion chamber. It is mounted with aninterference fit inside the body 8 and receives a pre-heating electrode(not shown). The core 12 transmits electrical energy to the electrodesituated in the finger 10 and, thereby, is in contact with thatelectrode and is connected to the finger 10 which it extends within thebody 8. At the opposite end of the finger 10, the free end of the core12 enables it to be electrically connected to a power conductor.

The pressure sensor is a piezo-electric sensor. It comprises apiezo-electric element 18 disposed between two contact elements 20 ofelectrically conductive material, the sensor itself being electricallyinsulated from the rest of the glow plug 2 by electrically insulatingmembers 22. The pressure sensor 14 comes to bear on the body 8 via abearing part 24. The other face of the sensor comes to bear against anut 26 fastened to the upper end of the core 12. The part of the glowplug 2 where the pressure sensor 14 is to be found and which is situatedoutside the engine is also termed glow plug head. At this head, the body8 has a gripping zone 28 used for the mounting/de-mounting of the glowplug 2 with respect to the cylinder head 6. Usually, this gripping zone28 is cylindrical and of hexagonal cross-section. In this way, using aspanner, and obtaining access from the exterior of the cylinder head 6,the mounting and the demounting of the glow plug 2 are possible. Thediameter of this gripping zone 28 is, in a conventional manner, greaterthan that of the rest of the body 8.

FIG. 2 represents an external view of a glow plug according to theinvention. With respect to the glow plug of FIG. 1, there is once againa body 8 having a screw thread 16 on its other surface. A gripping zone28 can also be recognized on the head of the body 8. At the other end ofthe glow plug is a finger 10′. In the embodiment represented in thedrawing it is a ceramic finger 10′. A first advantage of such a fingeris to have a longer life than the fingers generally encountered for glowplugs of the prior art. It is also more compact in form. Thus, thefinger 10′ is very appreciably smaller than the finger 10 of FIG. 1.

The glow plug of FIGS. 2 to 4 also comprises a pressure sensor 14. Heretoo this is a piezo-electric sensor which may be of the same type as thesensor of the glow plug represented in FIG. 1.

Whereas in FIG. 1 the finger 10 fits into the body 8, in the embodimentof FIGS. 2 to 4, in accordance with the invention, an intermediatetubular part 30 is provided between the body 8 and the finger 10′. Thisintermediate tubular part 30 is intended to isolate the finger 10′ fromthe body 8.

An important difference between the sensors of the prior art and thesensor according to the invention described here is that the pressuresensor is no longer disposed between the body 8 and the finger 10 but isdisposed between the finger 10′ and the intermediate tubular part 30.This is apparent in particular in the cross-section of FIG. 3.

In the following portion of the description, it will be considered thatthe members intended to be located at the combustion chamber side whenthe glow plug is fixed into the engine are at a lower level than themembers located outside the combustion chamber at the glow plug headside.

The finger 10′ has an interference fit with the lower end of theintermediate tubular part 30. In a conventional manner, only the upperportion of the finger 10′ is within the intermediate tubular part 30,the lower portion of the finger 10′ projecting outside the part whichcarries it so as to be situated within the combustion chamber and toheat the gaseous mixture which is located therein.

The core 12 supplying the finger 10′ with electrical energy passesthrough the entire intermediate tubular part 30.

The intermediate tubular part 30 is itself fixed with an interferencefit to the lower end of the body 8 at a fixing zone 32. The end of theglow plug is also sometimes referred to as glow plug cone. Above thefixing zone 32 of the intermediate tubular part 30 on the body 8, theintermediate tubular part 30 extends freely within the body 8. In itsupper portion, the intermediate tubular part 30 flares out and has azone of greater diameter serving as a housing 34 for the sensor 14. Inthe glow plug, that sensor 14 is thus located at the head of the glowplug, that is to say in the upper portion thereof.

The upper portion of this housing 34 is tapped so as to be able toreceive a nut having an outer screw thread. This nut 36 is provided topress on the upper face of the sensor 14. The lower face of the sensor14 rests on a spacer 38. The latter links the finger 10′ to the sensor14.

The sensor 14 is for example a sensor of the type described withreference to FIG. 1. It is not useful to repeat the description of thesensor here. The important different between the sensor 14 of FIG. 1 andthat of FIGS. 2 to 4 is that in FIG. 1 the sensor 14 works in tensionwhereas in FIGS. 2 to 4 is works in compression. This does not howevermodify its structure and in particular there is once again apiezo-electric element disposed between two contact elements to whichelectric wires are connected.

The spacer 38 is a tubular part to enable in particular the passage ofthe core 12 through its center. The outer diameter of the spacer 38 isadapted to the inner diameter of the intermediate tubular part 30 so asto extend freely therein. In its lower portion, the diameter of thespacer 38 is adapted to the diameter of the finger 10′. In its upperportion, the diameter of the spacer 38 is adapted to the diameter lowerface of the sensor 14. Thus, as may be noted on the drawing, the spacerhas a frusto-conical zone 40 in its upper portion making it possible toprogressively pass from the diameter corresponding to the finger 10′ tothat of the sensor 14.

The pressure sensor 14 solely measures here the stresses arising fromthe ambient pressure in the corresponding combustion chamber. Thevariants stresses exerted on the cylinder head 6 in which the glow plugis mounted, in particular those which are not proportional to theambient pressure in the combustion chamber, have no influence on thepressure measured by the sensor 14. This is because the intermediatetubular part 30 is held in the body 8 at a zone of the body which doesnot undergo the stresses linked to the deformation of the cylinder head6 into which the glow plug 2 is screwed. More particularly, thesedeformations are rather to be found at the screw thread 16.

The sensor 14 is mounted between the intermediate tubular part 30 andthe finger 10′. To obtain a correct measurement of the pressure exertedon the finger 10′, care is taken in the design of the glow plug toprovide a deformation zone 42 of the intermediate tubular part 30. Thisdeformation zone 42 is located between the zone in which the finger 10′has an interference fit inside the intermediate tubular part 30, and thezone in which the intermediate tubular part 30 fits into the body 8.This deformation zone 42 favors the transmission of the forces exertedby pressure on the finger 10′ of the sensor 14.

The role of the spacer 38 is important here and great attention must betaken in its manufacture. On passing on the forces exerted on the finger10′, the spacer 38 must not transmit parasitic forces. Care shouldtherefore be taken that the spacer is in contact neither with the core12 supplying the finger 10′ with electrical energy nor with theintermediate tubular part 30.

Care should also be taken that the spacer's own vibration mode does notperturb the pressure measurement of the sensor 14. Care must thereforebe taken that the own vibration frequency of that spacer 38 is far awayfrom the bandwidth of the sensor 14. In choosing a ceramic as materialfor producing that spacer 38, these problems may be solved.

As shown in FIGS. 3 and 4, the head of the glow plug may comprise anovermolded part 44 of synthetic material. This part may incorporate thegripping zone 28 of the glow plug head. The overmolding with syntheticmaterial makes it possible to ensure good fluid-tightness of the glowplug head and ensure good protection of the electrical connections ofthe electrical wires connected to the contact elements of the sensor 14.

A glow plug 2 comprising a pressure sensor 14 such as the glow plugdescribed above makes it possible to make an excellent pressuremeasurement within a combustion chamber. The measurement of thispressure is totally decoupled from the parasitic noise liable to perturbthe pressure measurement. Thus even where there are injectors situatedat each cylinder, the pressure sensor mounted in a glow plug accordingto the invention makes it possible to get free from the deformations ofthe cylinder head induced by the stresses exerted by those injectors. Asfor the deformation zone on the intermediate tubular part, this makes itpossible to obtain good sensitivity for the measurement made by thesensor.

As is apparent in particular from FIG. 4, despite the introduction of anintermediate tubular part, the glow plug according to the inventionremains easy to assemble. It is a matter of stacking parts which aresealed by an overmolding of plastics materials at the glow plug head.

The present invention is not limited to the preferred embodimentdescribed above by way of non-limiting example. It also concerns all thevariant embodiments accessible to the person skilled in the art.

Thus, for example, the present invention may perfectly well be envisagedwith a conventional finger. The drawing of the glow plug must then beadapted to that finger of greater size but it is of course stillpossible to introduce an intermediate tubular part to achieve thedecoupling between the body of the glow plug and the finger thereof.

The fixing of the finger in the intermediate tubular part andrespectively of the intermediate tubular part in the body is providedabove by interference fit. Another way of fixing (welding, for example)may be envisaged.

An alternative to the use of a spacer may be envisaged. There may forexample be provided a nut fastened to the core supplying the finger withelectrical energy and on which the lower face of the pressure sensorwould come to rest. To withstand the high mechanical forces to transmit,rigidifying that core may then have to be envisaged. If the spacer iskept, it may take a different form to that described above. That form isadvantageous since it makes it possible to have only one part betweenthe finger and the sensor. However, it would be possible to have aspacer associated with a washer for bearing upon which would form theinterface between the spacer and the sensor.

1. A glow plug (2) comprising: a tubular body (8) having means (16) forfixing into a bore; an intermediate tubular part (30) mounted in thetubular body (8); a finger (10′) arranged inside the intermediatetubular part (30) and configured to receive a heater electrode; and apressure sensor (14) having a first face connected to the intermediatetubular part (30), the first face facing away from the finger (10′),wherein the pressure sensor (14) has a second face that faces the finger(10′) and bears on the finger (10′).
 2. The glow plug (2) according toclaim 1, wherein the intermediate tubular part (30) is connected to thetubular body (8) adjacent to the end of the tubular body (8) adapted tobe in a combustion chamber of an engine (4).
 3. The glow plug (2)according to claim 1, wherein the intermediate tubular part (30) isfixed to the tubular body (8) by an interference fit.
 4. The glow plug(2) according to claim 1, wherein the intermediate tubular part (30) iswelded to the tubular body (8).
 5. The glow plug (2) according to claim1, wherein the finger (10′) is fixed to the intermediate tubular part(30) by an interference fit.
 6. The glow plug (2) according to claim 1,wherein the finger (10′) is welded to the intermediate tubular part(30).
 7. The glow plug (2) according to claim 1, wherein theintermediate tubular part (30) includes a deformation portion (42)between a first fixing portion in which the finger (10′) is connected tothe intermediate tubular part (30) and a second fixing portion (32) inwhich the intermediate tubular part (30) is connected to the tubularbody (8).
 8. The glow plug (2) according to claim 1, wherein a spacer(38) is arranged between the finger (10′) and the second face of thepressure sensor (14) inside the intermediate tubular part (30).
 9. Theglow plug (2) according to claim 8, wherein the spacer (38) is comprisesa ceramic material.
 10. The glow plug (2) according to claim 8, whereinthe spacer (38) comprises an elongate tubular bush with an outerdiameter less than an inner diameter of the intermediate tubular part(30), a terminal end of the tubular bush having a frusto-conical portion(40) adapted to come into contact with the pressure sensor (14) so as topresent a bearing surface adjacent to the pressure sensor (14)substantially corresponding to a surface of the second face of thepressure sensor (14).
 11. The glow plug (2) according to claim 1,wherein the pressure sensor (14) is placed in a housing (34) provided inthe intermediate tubular part (30) adjacent to a first end of thetubular part, the first end being opposite to a second end of theintermediate tubular part (30) adapted to receive the finger (10′), andwherein the pressure sensor (14) is connected to the intermediatetubular part (30) by a nut (36) having an outer periphery with a screwthread configured to engage with a tapping at the first end of theintermediate tubular part (30).
 12. The glow plug (2) according to claim2, wherein the intermediate tubular part (30) is fixed to the tubularbody (8) by an interference fit.
 13. The glow plug (2) according toclaim 2, wherein the intermediate tubular part (30) is welded to thetubular body (8).
 14. The glow plug (2) according to claim 9, whereinthe spacer (38) comprises an elongate tubular bush with an outerdiameter is less than the inner diameter of the intermediate tubularpart (30), a terminal end of the tubular bush having a frusto-conicalportion (40) adapted to come into contact with the pressure sensor (14)so as to present a bearing surface adjacent to the pressure sensor (14)substantially corresponding to the surface of a surface of the secondface of the pressure sensor (14).
 15. The glow plug (2) according toclaim 11, wherein the nut (36) connecting the pressure sensor (14) tothe intermediate tubular part (30) is arranged to press on the firstface of the pressure sensor (14).
 16. The glow plug (2) according toclaim 1, wherein the intermediate tubular part (30) extends through afirst opening at a first end of the tubular body (8) and out from asecond opening at an opposite second end of the tubular body (8). 17.The glow plug (2) according to claim 1, wherein the intermediate tubularpart (30) separates an entire inner surface of the tubular body (8) fromthe finger (10).